The invention described herein relates to nuclear reactors and more particularly to an improved method of fabricating an annular neutron absorbing pellet used in burnable poison rods in the reactor, together with an improved pellet composition having desirable operating characteristics.
The burnable poison rod of the type more recently developed for use in nuclear reactors includes a pair of concentrically disposed zircaloy tubes having an annular space therebetween filled with neutron absorbing pellets of cylindrical configuration. Each pellet comprises boron carbide in an aluminum oxide matrix formed into a two-inch long tube of thin wall construction of about 0.020 inch to 0.040 inch thereacross. Hollow end plugs seal the pellets in the annular space but reactor coolant is permitted to flow upwardly through the center tube and outside the outer tube during reactor operation. This design results in a reduction of fuel cycle cost and increased flexibility in core fuel management.
Investigations carried out during the course of making the present invention which relates to a burnable poison pellet as such, reaffirm that as the pellets are irradiated, they undergo dimensional changes of a magnitude sufficient to structurally deform the concentric tubes in which they are enclosed. The dimensional changes result from densification and swelling in the pellet body. Densification takes place as a result of removal of porosity by irradiation and thermal processes in a reactor. Swelling occurs as the B.sub.10 -neutron reaction produces solid fission products and helium gas which fill pores intentionally incorporated in the pellet body.
The hygroscopic characteristics of the aluminum oxide (Al.sub.2 O.sub.3) presents another disadvantage when used in the reactor. After manufacture, the Al.sub.2 O.sub.3 --B.sub.4 C pellets rapidly absorbs moisture which must be removed prior to being loaded and sealed in the zircaloy tubes' annular space, otherwise hydriding of the zircaloy tubes could later occur with possible tube failure. Since these pellets are of varying theoretical density, e.g., more than 60% theoretical density, and have relatively high internal surface areas, moisture absorption can range to values greater than 1,000 ppm. Although the pellets are readily dried by heating to about 300.degree. C., they nevertheless reabsorb moisture in a matter of minutes. Consequently, the pellets must be preserved in a very dry atmosphere after drying to prevent moisture reabsorption to levels in excess of the specification. Should atmospheres of this type not be available, and in absence of still other ways to control pellet moisture, the need then exists to protect the pellet against moisture infiltration by requiring that pellets be loaded into a burnable poison rod, including pressuring and end plug sealing, in a dry glove box environment or other arrangement where the pellets will not be exposed to humidity in the atmosphere. It is evident that fabrication of rods under these conditions would not only increase manufacturing cost substantially but would require rigorous quality control procedures to help assure the pellets have not reabsorbed excessive moisture after drying.
These and other disadvantages which will be apparent as the description proceeds, suggest that the need is great for burnable poison pellets designed to be stable in the operating environment of a reactor by being highly resistant to dimensional change, and which contain an equilibrium moisture content which will not significantly produce hydriding as the pellets are irradiated during reactor operation.